* Remove PR template, update guidelines for PR
* Update testing-guidelines.md
* Add clarification on when to use runtime/compiletime checks
* Update issue label descriptions
Rework of contributor docs to reflect actual flow
for both internal and external developers using
the PowerShell/PowerShell repo for the first time.
Includes updates to issue/PR guidelines, repo 101,
and CONTRIBUTING.md.
Zakaria Ridouh